1. Field of the Invention
This invention relates to a magnetic field compensating apparatus which compensates a radial direction component of an error magnetic field contained in a magnetic field produced by a main coil of a magnet device used for a magnetic resonance apparatus, and particularly to a small-sized, inexpensive and highly accurate magnetic field compensating apparatus.
2. Prior Art
In general, uniformity of magnetic field in a region in which an object to be examined is installed is one of important characteristics in a superconductive-type magnetic resonance apparatus. Accordingly, with respect to a main coil for generating an output magnetic field from a magnet device used for a magnetic resonance apparatus, various ideas are applied to its shape, its current distribution and the like in order to make a magnetic field highly uniform.
But, a highly uniform magnetic field is liable to be disturbed by internal conditions of the magnet such as accuracy in manufacturing and a temperature condition or environmental conditions such as a ferromagnetic substance disposed near the magnet. Until now, therefore, a magnetic field compensating apparatus for compensating an error magnetic field has been built in a magnetic field resonance apparatus.
The component B(X,Y,Z) of the error magnetic field can be represented by the following Maclaurin expansion of the output of the magnetic field at the center point of the magnetic field. ##EQU1## Here, B.sub.0 shows a necessary component of the uniform magnetic field, and B.sub.1 X, B.sub.2 Y, and B.sub.3 Z represent the first order error magnetic field components toward the X, Y, and Z directions, respectively. And the following of B.sub.1 X, B.sub.2 Y and B.sub.3 Z show components of higher order error magnetic field components in order.
In the case where the main magnetic field generated by the main coil is directed toward the Z direction, error magnetic field components containing X and/or Y (For example, B.sub.1 X, B.sub.2 Y, B.sub.1 X.sup.2, B.sub.5 Y.sup.2, B.sub.7 XY, B.sub.8 YZ, B.sub.9 ZX, --) are called radial direction error magnetic field components.
Until now, compensation of radial direction error magnetic field components has been performed for each component. Accordingly, when compensation is performed using, for example, coils, such a series of independent coils corresponding to each component as an X compensating coil (X-sim coil), a Y compensating coil (Y-sim coil), a XY compensating coil (XY-sim coil),--are employed.
FIG. 1 is a block diagram showing a X compensating coil in a conventional magnetic field compensating apparatus. In the diagram, numerals 11 through 14 show four saddle-shaped coils which are installed cylindrically and disposed along the Z direction. These coils 11 through 14 compose a family of coils of one unit, and they are series connected with each other to be driven by one power source (not shown). An arrow i, a character a, and a character .theta. show a current flowing in the coils 11 through 14, a radius of arc sections of the coils 11 through 14, and an angular aperture of the arc sections, respectively.
The actual magnetic field compensating apparatus has a multi-layer construction in which other cylindrical compensating coils (not shown) are piled up and disposed concentrically in order on the family of coils shown in FIG. 1 in order to compensate error magnetic field components in the above-mentioned other radial directions (For example, a Z.sup.2 X component, an X.sup.3 component, etc.). Since a family of coils for compensating the higher order components of the error magnetic field such as the Z.sup.2 X component, the X.sup.3 component, etc. are provided with 6 to 8 saddle-shaped coils, 18 or more saddle-shaped coils are usually needed in order to perform magnetic field compensation for three components containing the X component.
FIG. 2 is a sectional view, taken along the line 2--2 of FIG. 3, showing an example of a superconductive magnet around which the X compensating coil shown in FIG. 1 is actually disposed together with other compensating coils (Y, Z, ZX, XY, --). FIG. 3 is a sectional view looking in the direction of the arrows 3--3 in FIG. 2.
In the views, numerals 30, 31, and 32 are opening of a superconductive coil, a superconductive main magnet, and a compensating coil which is manufactured as a superconductive coil and installed concentrically with the superconductive main coil 31, respectively. The structure indicated by numerals 33, 34, and 35 includes a liquid helium tank 34 which contains the superconductive main coil 31 and the compensating coil 32 and comprises a thermal shield 33, liquid helium tank 34, and a vacuum tank 35 which covers the liquid helium tank 34. A numeral 36 is a compensating coil which is manufactured as an ordinary conductive coil and disposed in the opening 30. In some apparatus both compensating coils 32 and 36 are installed; in other apparatus only one of them is installed.
Next, there will be given description of compensating operation for an error magnetic field in the Z direction using the X compensating coil shown in FIG. 1. A magnetic field component which compensates the error magnetic field in the Z direction is generated by only the arc section among the arc section and the linear section which form each of the saddle-shaped coils 11 through 14. The expression which represents the magnetic field output B.sub.z (x,y,z), is as follows. ##EQU2## The expression (2) represents outputs of the first order, the third order and up to the fifth order, and neglects outputs of much higher orders.
In case of the X compensating coil, the first term of the first order in the expression (2) is an effective one, and the term of the third order, the term of the fifth order and terms of orders higher not less than the fifth order become errors for compensation. When the partial derivatives in the expression (2) are represented concretely, they can be represented by the radius a, the angular aperture .theta. of the arc section and the position in the Z direction where the arc section is disposed. In the X compensating coil, therefore, proper values are selected for the position in the Z direction and the angular aperture .theta. of the arc section, and the term of the third order (or terms of orders not less than the fifth order) in the expression (2) is (or are) eliminated as the total sum of the eight arc sections of respective saddle-shaped coils 11 to 14.
Accordingly, in order to make other components small, the position in the Z direction is limited, and at the same time, the angular aperture .theta. is limited and can not be made large. As a result, compensating magnetic field components per ampereturn are made small.
As described above, a conventional magnetic field compensating apparatus has had the following problems. Since it performs error magnetic field compensation in the radial direction for only one component (For example, an X component), the position in the Z direction of the arc section of the compensating coil, the angular aperture .theta. thereof, and the like are limited in order to make other components zero or sufficiently small. Accordingly, compensating magnetic field components per ampereturn are made small, thereby decreasing the degree of accuracy of the apparatus. Furthermore, in the compensating coil for higher order error components of the main coil, the number of the saddle-shaped coils and the number of layers thereof are increased, thereby making the apparatus larger in size and, at the same time, making it expensive.